Sulfation is a common modification that affects the biological activity of a wide variety of substrates. Sulfation reactions are catalyzed by sulfotransferases. Sulfotransferases are a large group of enzymes that transfer a sulfate from a donor substrate to an acceptor. Many sulfotransferases exist in nature, but in humans, the sulfotransferases are of two types. Cystosolic sulfotransferases (SULTs) are mainly involved in modifying small molecules such as steroids, neurotransmitters, and xenobiotics, and in drug detoxification. Golgi-resident sulfotransferases are involved in modifying glycans and proteins on cell membranes and within the extracellular matrix. Sulfated glycans such as glycosaminoglycans and numerous O- and N-glycans play roles in maintaining biochemical and biophysical properties. Sulfated proteins, such as leukocyte adhesion molecule PSGL-1, play roles in protein-protein and cellular interactions.
Because of the important roles of sulfated molecules in various biological events, sulfotransferases may be ideal targets for drug intervention. Several assays for detecting sulfotransferase activity exist, but they have significant drawbacks. Some sulfotransferase assays uses a radioisotope, 35S. Such assays require separation steps such as HPLC, TLC, filter blinding, or electrophoresis to separate substrates from products. As such, in addition to the difficulties associated with the use of radioisotopes, the need for separation makes such assays very time consuming. Other methods which can be used include mass spectrometry, fluorescent detection and colorimetric detection. One fluorescent method uses 4-methylumbelliferyl sulfate as the ultimate sulfate donor and measures the fluorescence of 4-methylumbelliferone. Similarly, one colorimetric detection method uses p-nitrophenyl sulfate as the ultimate sulfate donor and measures the color intensity of the generated p-nitrophenol. Each of these methods is either time consuming or lacks efficiency, limiting their usefulness. An assay which can be performed easily and provides results rapidly would be ideal, particularly for screening potential drugs for their effect on sulfotransferases.